In digital communications, it is often desirable for a plurality of users, or “nodes” to communicate over a shared, multiuser network, rather than relying on point-to-point communication. While point-to-point communication offers the advantage of guaranteed, dedicated access to a communication medium, and consequent high communication speeds, implementation costs for a point-to-point communication system can be prohibitively high, scalability is typically difficult, and configuration can be burdensome.
Shared, multiuser networks allow a plurality of users, or “nodes,” to share a common communication medium, thereby reducing costs and enhancing scalability and ease of configuration. Of course, a significant disadvantage of shared medium networks as compared to point-to-point networks is that they are much more limited in traffic capacity, due to the requirement that only one node transmit information on the shared communication medium at any given time. If more than one node attempts to transmit at the same time, a so-called “collision” occurs, whereby the transmitted information is unintelligible, and must be re-transmitted.
Shared networks are typically either “managed” or “ad hoc.” In a managed network, a central controller or “access point” manages use of the shared communication medium, thereby minimizing collisions and managing recovery from any collisions that do occur. In an “ad hoc” network, all nodes are treated equally, and must negotiate directly with each other so as to coordinate use of the shared communication medium, avoid collisions, and recover from collisions. Typical examples of ad hoc digital networks include Ethernet networks and wireless “802.11b” networks.
The decentralized nature of ad hoc networks makes them suitable for a variety of applications. They are naturally robust, since no single node is vital to the network, and they are easy to scale, since configuration requirements are typically minimal, and deployment is usually quick and automatic.
However, due to the lack of a central controller, ad hoc networks must use special protocols so as to avoid and recover from collisions. One commonly used protocol is the Carrier Sense Multiple Access protocol, or CSMA. In this approach, each node monitors the network so as to sense if any communication traffic is present. A node only attempts to transmit information when no traffic is detected. Collision avoidance can be further enhanced in wired networks by using a CSMA/CD protocol, which adds “Collision Detection” to the basic CSMA protocol. In CSMA/CD, a transmitting node monitors the network while it is transmitting, halts transmission if a packet collision is detected, and then repeats the attempt at a later time.
However, it is typically not possible for collision detection to be used in a wireless network, since it is generally not possible for a wireless node to monitor the network while it is transmitting. Hence, it is not generally possible for a wireless node to directly detect collisions during transmission, making it impossible for a wireless network to use collision detection protocols such as CSMA/CD. Also, due to distances and intervening structures, it may not be possible for a specific node in a wireless network to detect signals from all other wireless nodes on the network. The likelihood of inadvertent collisions in wireless ad hoc networks is therefore much greater than for wired networks.
So as to enhance avoidance of collisions on a wireless ad hoc network, a “Collision Avoidance” protocol such as CSMA/CA can be used. In the CSMA/CA approach, after determining that no traffic appears to be present on the network, a transmitting node first sends a signal to all other nodes instructing them not to transmit, and then transmits its packet of information. CSMA/CA can be further enhanced by the exchange of a Request to Send (RTS) packet sent by the sender, and a Clear to Send (CTS) packet sent in reply by the intended receiver. These packets serve to alert all nodes within range of the sender, the receiver, or both, to keep quiet for the duration of the main packet. This is known as the IEEE 802.11 RTS/CTS exchange.
As mentioned above, a significant disadvantage of shared medium networks as compared to point-to-point networks is that shared networks are much more limited in traffic capacity, due to the requirement that only one node transmit information on the shared communication medium at any given time. This requirement can be relaxed somewhat by using one or more so-called “spread-spectrum” technologies to “spread” the shared communications medium into a plurality of separately usable communications channels.
For example, Frequency Division Multiple Access, or FDMA technology, provides for transmissions to occur in different frequency bands on the shared communications medium. Filters are then used to isolate the transmissions on the separate frequency channels, allowing for simultaneous information transmission on each of the frequency-separated channels. Of course, FDMA requires an increased communication bandwidth, and so is limited in wireless applications according to bandwidth allocations.
Time Division Multiple Access, or TDMA, divides time into available channel “slots.” Nodes transmit in rapid succession, one after the other, in a repeating cycle, each node using its own time slot. This approach accelerates communication rates by minimizing the overhead required by negotiation protocols such as CSMA.
Code Division Multiple Access, or CDMA, allows a limited degree of shared use of a communications channel, wherein different nodes transmit using different encoding schemes that can be distinguished from each other by the receiving nodes.
Specifically in the case of wireless networks, Spatial Division Multiple Access, or SDMA, can also be used. One example of SDMA is the division of a geographic region into communication “cells” in a typical cellular telephone network.
Depending on the circumstances, some or all of the above spread-spectrum technologies can be combined so as to yield a further increase in communication speed. In general, however, even when spread-spectrum technologies are employed, the communication speed of a shared ad hoc wireless network is still significantly limited due to the requirement that only one node transmit over a single communication channel at any given time, and due to the added overhead of the various protocols that are used to avoid and/or recover from packet collisions.